According to Gruber’s sources, The Daily will be an app in the App Store, but make use of new recurring subscription billing on users’ iTunes accounts, and “developers at News Corp. building the app already have preliminary documentation on the new subscription billing APIs from Apple.”

According to Viticci’s sources, iOS 4.3 wasn’t intended to be released so quickly after 4.2.1, which was originally internally slated for an early November release. It’s possible that 4.2.1’s later official release might also push back the release of 4.3. But with Apple playing such a large role in The Daily, both companies may stick with mid-December announcement and releases, after all.

Subscription-based recurring billing would likely increase the number of paid magazine, newspaper, TV, video and other media applications on iTunes. Really, any application that depends on continuous content or service delivery could introduce a subscription model: online gaming, data backup, GPS, office applications and more. Many subscription-based services already have iOS apps, but have to establish accounts and recurring billing separately from iTunes.

Another technical challenge posed by subscriptions that could require an OS update is automatic background content delivery. If you’re being billed every week for a newspaper or magazine, you shouldn’t have to go through a long, complicated routine just to download a new issue.

A final open question: How much customer information will Apple and app/content makers share with each other about their subscribers? This data has value, too — as does customers’ privacy.